You can also use anchors like decide.east if you want to force the arrows to come out of the points of the decision rhombus. This isn't needed here but if you want a diagonal line coming out of the decision node it will by default come from the center and emerge from one of the sides.

I would like to allow other users to drop lines of text into the blocks
of a pre-constructed flowchart like this. I know the "box and arrow"
skeleton of the flowchart, so to speak, but not the number of text lines
in any particular block. (Every block will have at least one text
line.) In the code given here, if I put too many text lines in block
"A", that block will expand so as to clobber blocks below it. Is there
a way to modify this script such that blocks and arrows below "A" will
automatically "move down" to accomodate many lines of text in "A"?

Please ignore my previous two questions. BTW, I would have found a simple example of a flowchart that uses the \matrix command to be very useful. Most currently-available examples are pretty complex, at least for me.